Implantable cardiac stimulation devices, such as defibrillators and pacemakers, are used to treat patients suffering from cardiac arrhythmias or other cardiac dysfunction. In operation, such devices may continuously monitor the electrical activity of the heart and deliver appropriate stimulation therapies to normalize electrical or mechanical dysfunction. Cardiac stimulation devices must possess low self-discharge in order to have a useful life of many months, and must be highly reliable to provide a needed therapy whenever necessary. In implantable defibrillators, the power source must have a high rate capability to provide the necessary charge to the capacitors for delivering high-energy shocks. In addition, since cardiac stimulation devices are implanted, the battery must be able to supply energy from a minimum packaged volume.
Batteries or electrochemical cells are volumetrically constrained systems. The size or volume of components that go into a battery (cathode, anode, separator, current collectors, electrolyte, etc.) cannot exceed the available volume of the battery case. In addition, the appropriate amount of some components depends on the amount of other components that are used. These components must be “balanced” to provide discharge to the extent desired.
In a conventional cathode limited battery such as a lithium-silver vanadium oxide (LiSVO) battery commonly used in cardiac stimulation devices, the capacity (Q+) of the cathode must not exceed the capacity (Q−) of the anode. Cathode limited cells have been used in battery powered implantable medical devices such as heart pacemakers because of the proven reliability of their discharge over the long periods during which they are implanted. The volume occupied by the other battery components also depends on the cathode capacity (Q+) as reflected by the amount of cathode material in the battery. The amount of electrolyte depends on the amount of cathode material and the amount of it to be discharged since the cathode material swells as the battery is discharged and requires more electrolyte to fill the additional cathode volume. The volume of the separator and current collector depends on the area of the electrodes. The area of the electrodes depends on the area required for consistent pulse output as the battery is discharged. All of these components must be adjusted for a given battery volume.
One method for constructing a lithium anode cell is to wind cathode and anode elements together to form a cylindrical or oblong coil. In a coiled design, anode material is available on either side of the cathode windings and will deplete into the cathode as the battery is discharged. Reliable performance is assured by having an excess of anode capacity beyond the usable capacity desired of the cathode. This excess lithium is distributed through the length of the anode. The lithium winding forming the outermost winding of the coil has cathode material adjacent to its inner circumference but not on the outer circumference. Therefore the outermost winding of the anode need only be half the thickness of the inner windings. If an anode is constructed from a single piece of foil that is of uniform thickness, there is an additional excess of lithium on the outermost winding of the coil.
One disadvantage of a cathode-limited cell is that the excess lithium in the cell may reduce other materials after the cathode is fully discharged, which can lead to cell swelling. In order to prevent damage to circuitry within the implantable device surrounding the cell due to cell swelling, a reinforcing stainless steel plate may be provided against the cell. This stainless steel plate occupies valuable space in an implantable device and adds weight to the overall device. In regard to implantable medical devices, a reduced size and weight is desirable to ease the implant procedure and avoid patient discomfort at the implant site. Eliminating the excess lithium or the need for the stainless steel plate would allow the cell size, and therefore the overall size of the medical device, to be reduced. Alternatively, the volume occupied by excess lithium and the steel plate could otherwise be taken up by cathode material to increase the battery capacity.
Another disadvantage of a cathode-limited cell is that its resistance increases as a function of time after the cell is discharged to the second voltage plateau on its discharge curve. By limiting the amount of lithium and electrolyte material in the cell, the cell may be designed to utilize only the first voltage plateau. Superior long-term cell performance can be achieved since the same useful capacity can be provided as in a conventionally balanced cell but at a higher voltage toward the end of the discharge cycle. A lithium-limited balanced cell having these advantages is disclosed in U.S. Pat. No. 5,458,997 to Crespi et al, incorporated herein by reference in its entirety.
In a coiled, anode-limited cell it is desirable to reduce the excess anode material present in the outermost winding that does not interface cathode material on both sides. One method for manufacturing an anode-limited cell that eliminates the excess anode material involves layering two lithium foil pieces. One lithium foil is provided long enough to form all of the windings in a coil, and the other lithium foil is provided long enough to form only the inner windings. When the two foils are overlaid and coiled with a cathode, the inner coil windings are formed by a double layer of lithium foil and the outermost winding by a single layer of lithium foil.
Another method for reducing excess lithium in an anode-limited cell is disclosed in commonly assigned U.S. patent application Ser. No. 10/123,495 filed 15 Apr. 2002 and entitled, “Balanced Anode Electrode” invented by Aamodt et al., the contents of which are hereby incorporated by reference. The disclosed method utilizes a short, thin lithium foil joined to a long, thick lithium foil such that the shorter, thinner foil will form the outermost coil winding and the longer thicker foil will form all of the inner windings. This method advantageously provides a narrow tolerance of the anode material for fully gaining the benefits of an anode-limited cell.
The anode current collector is commonly provided as a perforated conductive material such as a nickel or titanium grid. During assembly, the anode current collector is pressed against the anode material such that the anode material becomes enmeshed with the openings in the anode current collector. In this way, the openings in the anode current collector grid act to strengthen the mechanical joint between the current collector and the anode material and prevent separation of these components. The anode current collector further acts to provide reinforcement to the thin lithium layer in the outermost winding.
A limitation exists, however, in coiled, anode-limited cells of this type relating to the potential for a short between the cathode material and the anode current collector. When the thin layer of lithium has substantially depleted into the cathode in the outermost winding, the cathode may press into the anode current collector potentially causing a short.
The anode current collector may extend the length of the coil windings or be positioned only on the outermost coil winding. The inventors of the present invention have discovered that, when the anode material becomes depleted toward the end of cell discharge, the cathode material may press against the anode current collector. Cracks that naturally occur in the cathode material create exposed edges of the cathode material that can press against the edges of the perforations or holes in the anode current collector grid, particularly in the final turn of the outermost winding of a coiled electrode assembly. Even if a separator material is present between the cathode and anode subassemblies, the separator material may tear when pressed between a cathode crack edge and an anode current collector edge. The possibility exists, therefore, for a short to occur between the cathode material and anode current collector causing the battery to fail unexpectedly.
Though this failure mechanism is expected to be highly infrequent, it could result in potentially serious consequences for a patient having an implanted device. Thus, an improved electrode assembly is needed which overcomes the potential failure mechanism described above, particularly in anode-limited cells but potentially in any cell in which cathode cracking may precipitate a short between the cathode and anode.